Given that there is no beginning, middle nor end to Slaughterhouse Five, I do not know that giving seven critical events in order will be much help.

However, the most important events in my opinion are:

Billy's first mention of the Tramalfadorian's. I believe that Billy experienced post traumatic stress disorder along with residual effects from his electroshock therapy. The aliens and his travel to Tramalfadore were a coping mechanism for Billy.

The passivity in which Billy lives his life. He walks on "blue and ivory feet." Essentially, he is the walking dead in his lack of passion for just about everything in his real life. Being in the war did not evoke lamentations about the horrors of war nor did being married to a woman he considered fat and unattractive.

The use of Paul Lazarro as a foil to Billy. Lazarro essentially bloomed as a solider and found himself very comfortable using violence and revenge every day.

Optometry. I find it ironic that Billy is an optometrist, a doctor that helps others to see better when he himself cannot see clearly.

Time travel. This concept in the book can be confusing if not looked at as Billy simply flashing back on his life. His memories are all he has and yet he barely lives in the present moment where he could be creating more memories.